332. M. Ampere also announced the fact of the attraction and repulsion of two wires conLecting the poles of a battery; and showed, that the magnetic needle, which had previously been used to prove the magnetic attractions and repulsions of the wire, could be replaced by another connecting wire like the first. This discovery seemed to liberate the phenomena of magnetism from any peculiar power resident in the magnet, and to prove its production by electricity alone. When by Oersted's discovery it had been shown that a wire connecting the poles of a Voltaic battery would act on a magnet, attracting and repelling it, just as another magnet would de, it was fair to assume that the wire possessed the powers of the magnet it supplied; and when the second magnet was replaced by another connecting wire, as in Ampere's experiment, and the powers and actions still remained as before, it was perfectly correct to consider these powers and actions as magnetical; so that it became evident that magnetism could be exerted independent of magnets, as they are usually called, and of any of the means of excitation usually employed, but wholly by electricity, and in any good electrically conducting medium.

333. The phenomena with two conductors situate between the poles of the battery are as follows:-When they are parallel to each other, and the same ends of them are similarly related to the battery; viz. when the supposed currents existing in them are in the same direction, then they attract each other; but if the opposite ends be connected with the battery, so that the currents conceived to exist in them are in opposite directions, they repel each other. If also, the one being fixed the other moveable, the currents be sent, or the connexions be made, in opposite directions, then the moveable one will turn round until they are in the same direction. The contrast between these attractions and repulsions, and those usually called electrical, are very striking. The one take place only when the circuit is completed: the other only when it is incomplete. The attractions take place between the similar ends of the wires, and the repulsions between the dissimilar ends; but the electrical attractions take place between dissimilar ends, and the repulsions between similar ends. These take place in vacuo, but those do not. When the magnetic attraction brings the two wires together, they remain in contact; but when electrical attraction brings two bodies together, they separate after the contact.

334. These experiments were varied in several ways by M. Ampere; and the apparatus with which they were made appears, from the plates and description published, to be very delicate, ingenious, and effectual. The general results drawn up by M. Ampere himself from them are, (1.) That two electrical currents attract when they move parallel to each other, and in the same direction; and repel when they move parallel to each other in a contrary direction. (2.) That when the metallic wires, traversed by these currents can only turn. in parallel planes, each of the currents tends to direct the other into a situation in which it shall be parallel, and in the same direction. (3.) That these attractions and repul

sions are entirely different from the ordinary electrical attractions and repulsions.

335. M. Arago stated to the Royal Academy of Sciences that he had ascertained the attraction of iron filings by the connecting wire of the battery exactly as by a magnet. This fact proved not only that the wire had the power of acting on those bodies already magnetised, but that it was itself capable of developing magnetism in iron that had not previously been magnetised. When the wire in connexion with the poles of the battery was dipped into a heap of filings, it become covered with it, increasing its diameter to the size of a goose-quill; the instant the communication was broken at either pole, the filings dropped off; and when it was re-established, they were re-attracted. This attraction took place with wires of brass, silver, platina, &c., and was so strong as to act on the filings when the wire was brought near them without actual contact. It was shown not to belong to any permanent magnetism in the wire or filings by the inactivity of both when the connexion was not made with the battery; and it was proved not to be electrical attraction by the connecting wire having no power over filings of copper, or brass, or over saw-dust. When soft iron was fused, the magnetism given was only momentary; but on repeating the experiment, with some modification, M. Arago succeeded completely in magnetising a sewingneedle permanently.

336. The theory which M. Ampere had formed, to account for the magnetic phenomena of magnets by electrical powers only, assumed that magnets were only masses of matter, around the axes of which electrical currents were moving in closed curves. This theory led him, when informed by M. Arago of his experiments, to expect a much greater effect if the connecting wire were put into the form of a spiral, and the piece to be magnetised were placed in its axis. According to the theory, in a needle or magnet, ponting to the north, the currents were in the upper part from east to west. In consequence of these expectations, M.M. Ampere and Arago made experiments with spirals or helices, and the results are mentioned in M. Arago's paper, on the communication of magnetism to iron filings in the fifteenth volume of the Annales de Chimie.

337. On twisting a wire round a rod, it may be made to pass either in one direction or the other, giving rise to two distinct but symmetrical helices, which have been named by botanists dextrorsum and sinistrorsum. Though their diameters be equal, and the spirals which compose them have equal inclinations, yet they can never be superposed; for, however they are turned about, their direction is the same. The dextrorsum, or, as we may call it, the right helix, proceeds from the right hand downwards towards the left above the axis; the tendrils of many plants exhibit instances of it, and it is almost exclusively used in the arts: the sinistrorsum, or left helix, proceeds from the left hand downwards towards the right above the axis.

338. Having made some of these helices, one

was connected by its extremities with the poles of a Voltaic hattery, and then a needle wrapped in paper placed within it; after remaining there a few minutes, it was taken out, and found to De strongly magnetised; and the effect of a helix above that of a straight connecting wire was found to be very great.

339. Then, with regard to the position of the poles in the magnetised needle, it was found that, whenever a right helix was used, that end of the needle towards the negative end of the battery pointed to the north, and that towards the positive end toward the south; but that with a left helix, that end of the needle towards the positive, pointed north; and the other end south. 340. In order to establish this point, the connecting wire was sometimes formed into one helix, sometimes into two or three, which was readily done by twisting it round a glass tube, or rod, first in one direction, then in another; and when needles previously enclosed in glass tubes were then placed in these helices, the magnetic poles they received were always in accordance with the statement just given. In one case, also, where the connecting wire had been formed into three consecutive helices, the middle one being of course different to the other two, a single piece of steel wire sufficiently long to pass through all three of them being enclosed in a glass tube was placed within them. On being again removed and examined, it was found to have six poles; first, a north pole, a little further on a south pole, then another south pole, a north pole, another north pole, and at the further end a south pole.

341. M. Boisgeraud read a paper to the Royal Academy of Sciences, containing the detail of numerous experiments, most of which, however, are variations of Oersted's first experiments. He remarked, that connecting wires, or arcs, placed any where in the battery, would affect the needle, a result that follows as a consequence from Oersted's and Ampere's experiments. He notices the difference of intensity in the effects produced when bad electrical conductors were employed to complete the circuit, a difference which Oersted himself had pointed out in the case of water. M. Boisgeraud, however, proposed to ascertain the conducting power of different substances by placing them in one of the arcs, cells, or divisions of the battery, and bringing the magnetic needle, or Ampere's galvanometer, towards another arc, viz. to the wire, or other connecting body, used to complete the circuit in the battery. With regard to the positions, M. Bonsgeraud's notices of the needle and wire, they are all confirmatory of Oersted's statements.

342. M. Ampere read another memoir on the phenomena of the Voltaic pile, and on the method he intended to pursue in calculating the action of two electrical currents. He also showed the mutual action of two rectilinear electrical currents; viz. of two straight portions of the connecting wires; for it appears that the phenomena of attraction, repulsion, &c., were first observed with spiral wires. These actions, however, are exactly similar; and the view already given of them, as it relates to straight wires, is consequently more simple than the de

scription of the effects with spiral wires can be; viz. considering it as a matter of experiment only, and not of theory.

343. In consequence of the view which M. Ampere had taken of the nature of magnetism as dependent simply upon currents of electricity, it became an important object with him to ascertain the action of the earth upon such currents excited by the Voltaic battery; for, from his theory, he expected that it would be equally efficient in directing these currents as in directing those supposed to exist in the magnetic needle. After some trials, he succeeded in overcoming the obstacles to delicate suspension, contact, &c., and constructed an apparatus in which a part of the wire connecting the two poles of a battery was rendered so light and mobile as to move immediately; the connexion was completed with the pole, and took a direction which, with regard to the earth, was always constant, and in accordance with M. Ampere's theory. An account of these experiments, with the apparatus used in them, was read to the Royal Academy. The first consisted of a wire bent so as to form almost a complete circle of about sixteen inches in diameter; the two extremities were made to approach, and were placed one just beneath the other; and being attached to two steel points, were connected by them with two little basins of platina containing mercury, fixed so as to receive them; only one of the points touched the bottom of the cup in which it was placed; so that the friction was scarcely any, and the mercury secured a good contact. The cups were connected with other wires that passed off to the Voltaic battery; so that it was easy to make this moveable circle connect either one way or the other between the poles; and being enclosed in a glass case, any movement it might receive was readily observed without danger of its resulting from any other cause than the electric action.

344. When the extremities of this apparatus were connected with the poles of a battery, the circle immediately moved, and after some oscillations placed itself in a plane perpendicular to the magnetic meridian of the earth; and, on every repetition of the experiments, the same effect took place. The direction in which it moved depended upon the way in which the connexion had been made with the battery; and if it be assumed that there is a current passing through the wire, from the positive to the negative end, the curve so arranged itself that that current always passed downwards on the eastern side, and upwards on the west. This circle moved round a perpendicular, and, therefore, only represented the direction of the magnetic needle: in order to represent the dip, a wire was formed into a parallelogram, and being fixed to a glass axis was suspended by fine points, and connected as before, so as to move round a horizontal axis, then this axis being placed perpendicular to the magnetic meridian, and the wires being connected with the poles of a battery, the parallelogram immediately moved towards a position in the plane perpendicular to the dipping-needle; when the communication was broken, it returned towards its first position; and when renewed, it resumed the second, evidently indicating the

magnetic influence of the earth over it. In consequence of the difficulty of placing the centre of gravity in the centre of suspension, and keeping it there, this conductor did not take its position exactly in a plane perpendicular to the dipping needle, but approached towards it till in equilibrium between the magnetic and the gravitating power of the earth.

345. M.M. Biot and Savart read a memoir to the Academy of Sciences, the object of which was to determine the law by which a connecting wire acted on magnetised bodies. Small rectangular plates, or cylindrical wires, of tempered steel, were made magnetical by the double touch, and being then suspended by silk-worm threads were placed in differed positions with, and at different distances from, the wire connecting the poles of the battery. The terrestrial magnetism was sometimes combined with that of the wire, sometimes opposed to it, and sometimes neutralised by the vicinity of another magnet. The different positions of equilibrium, and the number of oscillations of the needles, were then observed, and data gained, by which M.M. Biot and Savart were conducted to the following result, which expresses the action exerted by a molecule of austral or boreal magnetism, placed at any distance from a very fine and indefinite cylindrical wire, rendered magnetic by the Voltaic current. Let a line pass from this molecule perpendicularly to the axis of the wire, the force which draws the molecule is perpendicular to this line and to the axis of the wire; its intensity is reciprocal to the distance. The nature of the action is the same as that of a magnetised needle placed on the surface of the wire in a direction determinate and constant in its relation to the direction of the Voltaic current; so that a molecule of boreal magnetism, and a molecule of austral magnetism, would be drawn in different directions, though constantly according to the preceding expression.

346. M. Ampere noticed an effect produced by the connecting wire bent into a helix. This may be easily understood from considering that the direction of the magnetic power is always perpendicular to the conducting wire. When, therefore, the conducting wire is parallel to the axis of the helix, the power is perpendicular to that axis; when the wire forms a circle round the axis, in a plane perpendicular to it, the power is in the direction of the axis; but when, as in the helix, it passes round the axis in a direction intermediate between parallelism and perpendicularity, the direction of the power is of course inclined accordingly. In this case the power may be considered as composed of two portions, one perpendicular to the axis, the other parallel to it. As M. Ampere considered magnets to be assemblages of currents perpendicular to their axes, he wished, in his imitation of them, to do away with that effect due to the extension of the wire in the direction of the axis of the helix, and succeeded in this by making the wire at one end return through the helix so as not to touch it in any part; for, in this position, its magnetic effects being contrary to those belonging to the length of the helix, and also near to them, they neutralised or hid each other. An imitation of

a magnet was made by torming a helix, and making the wires at the two extremities return through the centre of the helix half way, and then pass out upwards and downwards, so as to form a perpendicular axis on which the whole might move. The extremity of a battery being connected with these two ends of the wire, the helix became magnetical, and was attracted and repelled by a magnet precisely as a real magnet would have been.

347. M. Biot's examination of the effects of magnetism, as impressed by electricity in motion, is too excellent to be passed unnoticed in our chronological account of discoveries in this science.

348. When the electric current, evolved from a Voltaic battery, is transmitted through any metallic bodies whatsoever, it gives them instantaneously a magnetic virtue; they become then capable of attracting soft and unmagnetised iron. This curious fact was discovered by M. Oersted. If we expose to these metallic bodies, a magnetic needle, they attract one of its poles, and repel the other, but only relative to the parts of their surface to which the needle is presented. Needles of silver or copper are not affected, but merely those susceptible of being magnetised. These effects subsist only under the influence of the electrical current. If we suspend the circulation of the electricity, by breaking off the communications established between the opposite poles of the Voltaic apparatus, or even if we retard considerably its velocity, by joining its poles with bad conductors, the magnetic power instantly ceases, and the bodies which had received it return to their usual state of indifference.

349. This simple sketch already displays many new properties. All the processes hitherto employed to magnetise bodies had produced an effect on only three pure metals, iron, nickel, and cobalt, and on some of their compounds, steel for example, which is merely a carburet of iron. Till now it was never possible to render silver, copper, or the rest of the metals magnetic. But the electric current gives all of them this property; it bestows it transiently by its presence; and, as we shall presently see, it diffuses it through the whole mass, in a manner equally singular, and which has no resemblance to what is produced, when we develope magnetism by our ordinary processes, which consist in longitudinal friction with magnetic bars.

350. To produce these novel phenomena in the simplest manner, we must, with M. Oersted, establish a communication between the two extremities of the Voltaic apparatus, by a simple metallic wire, which may be easily directed and bent in all directions. We place afterwards, in the neighbourhood of the battery, a very sensible magnetic needle, horizontally suspended. As soon as this is settled in the direction due to the magnetic force of the terrestrial globe, we take a flexible portion of the conducting, or conjunctive wire, as M. Oersted calls it, and having stretched it parallel to the needle, we bring it gently near it, either from above, from below, from the right, or from the left. We shall see an immediate deviation of the needle; but, what is not the least remarkable circumstance, the direction of this

deviation differs according to the side by which the conjunctive wire approaches it. Duly to comprehend this astonishing phenomenon, and to fix its peculiarities with precision, let us suppose that the conjunctive wire is extended horizontally from north to south, in the very direction of the magnetic direction in which the needle reposed, and let the north extremity be attached to the copper pole of the trough, the other being fixed to the zinc pole. Imagine, also, that the person who makes the experiment looks northward, and consequently towards the copper or negative pole. In this position of things, when the wire is placed above the needle, the north pole of the magnet moves towards the west; when the wire is placed underneath, the north pole moves towards the east; and, if we carry the wire to the right or the left, the needle has no longer any lateral deviation, but it loses its horizontality. If the wire be placed to the right hand, the north pole rises; to the left, its north pole dips; and in thus transporting the conjunctive wire all around the needle, in directions parallel to one another, we merely present it to the needle, by the different sides of its circular contour, without affecting in the least the proper tendency of the needle towards the terrestrial magnetic poles. Since then the deviations observed in these successive positions are first of all directed from right to left, when the wire is above the needle; then from above downwards, when the wire is to the left; from the left to the right, when the wire is beneath; and, finally, from below upwards when it is to the right hand, we must necessarily conclude from these effects, that the wire deranges the needle, by a force emanating from itself, a force directed transversely to the length of its axis, and always parallel to the portion of its circular contour, to which the needle is opposite. M. Oersted drew this inference from his first observations.

351. Now this revolutive character of the force, and revolutive according to a determinate direction, in a medium which like silver, copper, or other pure metal, seems perfectly identical in all its parts, is a phenomenon very remarkable, of which we had heretofore only one singular example in the deviations which certain bodies impress on the planes of polarisation of the luminous rays. The first fact of the magnetism, transiently impressed upon the conjunctive wire by the Voltaic current, might have offered itself to a vulgar observer. I do not know whether some traces of this property may not have been previously perceived and indicated; but to have recognised this peculiar character of the force, and to have delineated it, agreeably to its phenomena, without hesitation and uncertainty, is the praise which truly belongs to M. Oersted, and which constitutes a condition entirely new in the movement of electricity.

352. As soon as this beautiful discovery was known in France, England, and Germany, it excited the most lively sensation among men of science. One of our colleagues, in particular, M. Ampere, ardently verified it in all its cireumstances. Seizing with sagacity the revolutive character of the force impressed on the conjunctive wire, he directed it with judgment, and

skilfully developed the consequences which flowed from this property. His researches, which preceded those of the other French philosophers, have considerably occupied the Academy; but as the order of exposition, occasioned by the mutual dependence of the phenomena, hinders me from beginning with them, I have endeavoured to compensate for this inversion by rendering justice at once, to labors which have anticipated and facilitated others.

353. In the above experiments which M. Oersted had made, the conjunctive wire is presented to steel needles, previously magnetised. It may be asked, if the action then exercised is proper to the conjunctive wire, as the action of a bar of steel tempered and magnetised is proper to this bar, or if the action is communicated to the wire by the presence of the magnetic needle, as we see soft iron, which exercises no magnetic power of itself, acquire transiently this power in the presence of magnets? To decide this question it was necessary to examine whether a body, not magnetic in itself, but capable of becoming so by influence, soft iron for example, would experience a sensible action at the approach of a conjunctive wire, traversed by the Voltaic current. This was effected by M. Arago, who showed that filings of iron are attracted by these wires; a simple but important fact, which defines clearly one of the characters of the force by which the phenomenon is produced.

354. The first thing which we must determine, is the law according to which the force emanating from the conjunctive wire decreases at different distances from its axis. This enquiry has been the object of experiments which I made along with M. Savart, already known to the Academy by his ingenious discoveries in acoustics. We took a small needle of magnetised steel in the form of a parallelogram, and, to ensure its perfect mobility, we suspended it under a glass bell, by a single fibre of the silkworm, and gave it at the same time a horizontal direction. Then, in order that it might be entirely at liberty to obey the force emanating from the conjunctive wire, we screened it from the action of the magnetism of the earth, by placing a magnetised bar at such a distance, and in such a direction, that it exactly balanced this action. Our needle was thereby placed in the same freedom of movement as if there did not exist any terrestrial globe, or as if we had been able to transport ourselves with it to a great distance in space. We now presented it to a conjunctive cylindrical wire of copper, stretched in a vertical direction, and to which we had given such a length, that its extremities necessarily bent in order to attach them to the poles of the electric apparatus, should have, in consequence of their distance, so feeble an action on the needle that it might be neglected with impunity. This disposition represented therefore the effect of an indefinite vertical wire, acting on a horizontal and independent magnetic needle. As soon as the communication of the Voltaic current was completed, the needle turned transversely to the axis of the wire, conformably to the revolutive character indicated by M. Oersted; and it set itself to oscillate around this direction, as a